Braking system with power brake, braking force proportioning, anti-skid, and traction control functions

Abstract

A braking system having a pressure modulator which, during normal braking operation, delivers a fluid pressure which is a predetermined multiple of a fluid pressure generated by the master cylinder to a wheel cylinder. The pressure modulator has a solenoid actuator which is energized during braking force proportioning and anti-skid operations to reduce the delivery pressure. In a preferred embodiment, the pressure modulator further includes another solenoid actuator for increasing the delivery pressure during braking force proportioning operation and for delivering fluid pressure during traction control operation.

We claim: 
     
       1. A braking system for providing an anti-skid function and a braking force proportioning function for a wheeled vehicle, wherein, during anti-skid operation, a fluid pressure communicated to a wheel cylinder of a brake is reduced upon occurrence of a potential wheel skid condition to eliminate the potential wheel skid condition, whereupon said fluid pressure is increased to retard the rotation of the wheel, and wherein, during a braking force proportioning operation, the fluid pressure communicated to the wheel cylinder is controlled in response to a varying load condition imposed upon the wheel or to a varying road surface condition to which the wheel is subjected, said braking system comprising: the brake, including the wheel cylinder, responsive to a fluid pressure for retarding the rotation of the wheel;   a master cylinder actuated by a vehicle operator for generating a controlled fluid pressure;   a source of a predetermined fluid pressure substantially higher than said controlled fluid pressure;   a reservoir for a brake fluid; and,   a pressure modulator in fluid communication with said wheel cylinder, said master cylinder, said source and said reservoir for controlling fluid pressure communicated to said wheel cylinder;   said modulator comprising valve means responsive to said controlled fluid pressure for controlling communication between said source and said wheel cylinder and between said wheel cylinder and said reservoir so that a modulated fluid pressure which is a multiple of said controlled fluid pressure is communicated to said wheel cylinder during normal braking operation;   said modulator also comprising electronically controllable first biasing means responsive to electric control signals for biasing said valve means in such a manner that said modulated fluid pressure communicated to the wheel cylinder is reduced during anti-skid and braking force proportioning operations of the braking system.   
     
     
       2. A braking system according to claim 1, further comprising pressure summation means for receiving the controlled fluid pressure from the master cylinder and the modulated fluid pressure from the pressure modulator and communicating to said wheel cylinder the sum of said controlled and modulated fluid pressures. 
     
     
       3. A braking system according to claim 2, wherein said pressure summation means comprises a body and a double-headed piston movably received therein, said body and said piston cooperating together to define therebetween a controlled pressure chamber in fluid communication with said master cylinder, a modulated pressure chamber normally in fluid communication with said pressure modulator, an output pressure chamber in fluid communication with said wheel cylinder, and a low pressure chamber normally in fluid communication with said reservoir, and wherein said braking system further comprises an electronically controllable switching valve which communicates said pressure modulator with said modulated pressure chamber when deenergized but communicates said pressure modulator with said low pressure chamber when energized, whereby upon energization of the switching valve said pressure summation means communicates to said wheel cylinder a reduced fluid pressure. 
     
     
       4. A braking system according to claim 1, for further providing a traction control function wherein upon occurrence of a potential wheel spin condition the fluid pressure is communicated to the wheel cylinder to eliminate the potential wheel spin condition, and for increasing the braking force of the brake during the braking force proportioning operation in response to an increasing load condition imposed upon the wheel, said pressure modulator further comprising electronically controllable second biasing means responsive to electric control signals for biasing said valve means in such a manner that the fluid pressure is communicated to the wheel cylinder during the traction control operation and that the modulated fluid pressure communicated to the wheel cylinder is increased during the braking force proportioning operation. 
     
     
       5. A braking system according to claim 4, further comprising pressure summation means for receiving the controlled fluid pressure from the master cylinder and the modulated fluid pressure from the pressure modulator and communicating to said wheel cylinder the sum of said controlled and modulated fluid pressures. 
     
     
       6. A braking system according to claim 5, wherein said pressure summation means comprises a body having therein a modulated pressure chamber in fluid communication with said pressure modulator, an output pressure chamber in fluid communication with said wheel cylinder, a controlled pressure chamber in fluid communication with said master cylinder, a low pressure chamber in fluid communication with said reservoir, and a pressure release chamber in fluid communication with said pressure modulator; said pressure summation means also comprising a first piston movably disposed between said output pressure chamber and said modulated pressure chamber and biased toward said modulated pressure chamber, a second piston movably disposed between said controlled pressure chamber and said low pressure chamber and operable to be brought into engagement with said first piston in response to the controlled fluid pressure in said controlled pressure chamber, and a third piston movably disposed between said output pressure chamber and said pressure release chamber and biased toward said output pressure chamber;   said braking system further comprising a normally-closed shut-off valve disposed between said pressure modulator and said variable volume chamber, said shut-off valve operating to open communication between said pressure modulator and said variable volume chamber during anti-skid operation of the braking system whereby the fluid in said pressure release chamber is released causing said third piston to move toward the pressure release chamber thereby allowing the output pressure chamber to expand and lower the fluid pressure communicated to the wheel cylinder.   
     
     
       7. A braking system according to claim 6, further comprising a normally-open pressure cut-off valve disposed between said master cylinder and said controlled pressure chamber of said pressure summation means, said pressure cut-off valve operating during anti-skid operation of the braking system to close communication between said master cylinder and said controlled pressure chamber to prevent a kick-back of a brake pedal. 
     
     
       8. A braking system according to claim 7, wherein said wheel cylinder is also in fluid communication with the master cylinder, and wherein said braking system further comprises normally-open shut-off valve means responsive to said controlled fluid pressure for interrupting fluid communication between the master cylinder and the wheel cylinder during operation of the braking system and permitting fluid communication therebetween during non-operation of the system to relieve the fluid pressure at the wheel cylinder and/or to allow the brake fluid to flow into the wheel cylinder. 
     
     
       9. A braking system according to claim 5, wherein said pressure modulator is also in fluid communication with said wheel cylinder and said wheel cylinder is also in fluid communication with said reservoir, and wherein said braking system further comprises; electronically controllable, normally open, first shut-off valve means for permitting fluid communication between the pressure summation means and the wheel cylinder during normal braking and braking force proportioning operations and interrupting the fluid communication therebetween during anti-skid and traction control operations,   electronically controllable, normally closed, second shut-off valve means for controlling fluid communication between the pressure modulator and the wheel cylinder so that the modulated fluid pressure from said pressure modulator is communicated to the wheel cylinder to increment the fluid pressure at the wheel cylinder to eliminate the wheel spin condition during traction control operation or to retard the rotation of the wheel during anti-skid operation, and,   electronically controllable, normally closed, third shut-off valve means for controlling fluid communication between the wheel cylinder and the reservoir so that the wheel cylinder is communicated with the reservoir to decrement the wheel cylinder pressure to restore the rotation of the wheel during traction control operation or to eliminate the wheel skid condition during anti-skid operation.   
     
     
       10. A braking system according to claim 9, further comprising pressure transmission means for transmitting the modulated fluid pressure from said pressure modulator to said wheel cylinder while preventing the brake fluid from said pressure modulator from flowing into the wheel cylinder. 
     
     
       11. A braking system according to claim 10, further comprising electronically controllable mode change-over valve means for causing said valve means of said pressure modulator to respond to said predetermined fluid pressure thereby to communicate said predetermined fluid pressure to said wheel cylinder upon occurrence of the potential wheel spin condition to eliminate the potential wheel spin condition. 
     
     
       12. A braking system according to claim 5, further comprising electronically controllable mode change-over valve means for causing said valve means of said pressure modulator to respond to said predetermined fluid pressure thereby to communicate said predetermined fluid pressure to said wheel cylinder upon occurrence of the potential wheel spin condition to eliminate the potential wheel spin condition, said pressure modulator being also in fluid communication with said wheel cylinder, and said wheel cylinder being also in fluid communication with said reservoir,   said braking system further comprising;   electronically controllable switching valve means for permitting fluid communication between the pressure summation means and the wheel cylinder but interrupting fluid communication between the pressure modulator and the wheel cylinder during normal braking and braking force proportioning operations and interrupting fluid communication between the pressure summation means and the wheel cylinder but permitting fluid communication between the pressure modulator and the wheel cylinder during anti-skid and traction control operations,   electronically controllable, normally-open shut-off valve means for controlling fluid communication between the pressure modulator and the pressure summation means in such a manner that the modulated fluid pressure from said pressure modulator is communicated to the pressure summation means to increment the fluid pressure at the wheel cylinder to eliminate the wheel spin condition during traction control operation or to retard the rotation of the wheel during anti-skid operation, and,   electronically controllable, normally closed shut-off valve means for controlling fluid communication between the wheel cylinder and the reservoir in such a manner that the wheel cylinder is communicated to the reservoir to decrement the wheel cylinder pressure to restore the rotation of the wheel during traction control operation or to eliminate the wheel skid condition during anti-skid operation.   
     
     
       13. A braking system according to claim 12, wherein said pressure summation means comprises a body having therein a modulated pressure chamber in fluid communication with said pressure modulator, an output pressure chamber in fluid communication with said wheel cylinder, and a controlled pressure chamber in fluid communication with said master cylinder, said pressure summation means also comprising a first piston movably disposed between said output pressure chamber and said modulated pressure chamber and biased toward said modulated pressure chamber, and a second piston movably disposed in said controlled pressure chamber and operable to be brought into engagement with said first piston in response to the controlled fluid pressure in said controlled pressure chamber.   
     
     
       14. A braking system according to claim 4, wherein said wheel cylinder is also in fluid communication with said master cylinder and wherein said braking system further comprises pressure-responsive normally-open shut-off valve means responsive to the modulated fluid pressure from said pressure modulator for allowing said fluid communication between said master cylinder and said wheel cylinder in the absence of said modulated fluid pressure and interrupting said fluid communication in the presence of said modulated pressure. 
     
     
       15. A braking system according to claim 14, further comprising pressure transmission means for transmitting the modulated fluid pressure from said pressure modulator to said wheel cylinder while preventing the brake fluid from said pressure modulator from flowing into the wheel cylinder. 
     
     
       16. A braking system according to claim 15, further comprising a variable volume chamber device, said device having a variable volume chamber defined by a spring biased plunger and being in fluid communication with said master cylinder, the volume of said chamber varying in response to the magnitude of the controlled fluid pressure to permit the brake fluid from the master cylinder to flow into and out of the variable volume chamber whereby substantial travel of a brake pedal is provided to allow the vehicle operator to press down the brake pedal when said shut-off valve is closed. 
     
     
       17. A braking system according to claim 4, wherein said wheel cylinder is also in fluid communication with said master cylinder, said braking system further comprising switching valve means responsive to a differential pressure between said predetermined fluid pressure and said controlled fluid pressure for communicating the modulated fluid pressure from the pressure modulator to the wheel cylinder when said predetermined fluid pressure is higher than said controlled pressure and communicating the controlled fluid pressure from the master cylinder to the wheel cylinder when said predetermined fluid pressure is lower than said controlled fluid pressure.   
     
     
       18. A braking system according to claim 17, further comprising pressure transmission means for transmitting the modulated fluid pressure from the pressure modulator to the wheel cylinder while preventing the brake fluid from the pressure modulator from flowing into the wheel cylinder. 
     
     
       19. A braking system according to claim 18, wherein said wheel cylinder is also in fluid communication with a fluid reservoir of the master cylinder, said braking system further comprising normally open cut-off valve means cooperating with said pressure transmission means for permitting fluid communication between the wheel cylinder and the fluid reservoir of the master cylinder during non-operation of the braking system to relieve the fluid pressure at the wheel cylinder and/or to allow the brake fluid to flow into the wheel cylinder, said cut-off valve means cooperating with said pressure transmission means during operation of the system to interrupt fluid communication between the wheel cylinder and the master cylinder reservoir. 
     
     
       20. A braking system according to claim 18, further comprising normally open first cut-off valve means cooperating with said pressure transmission means for permitting fluid communication between the master cylinder and the wheel cylinder during non-operation of the braking system and interrupting fluid communication therebetween during operation of the system, said braking system further comprising normally open second cut-off valve means responsive to said controlled fluid pressure to interrupt fluid communication between the master cylinder and the wheel cylinder. 
     
     
       21. A braking system for providing a braking force proportioning function for a vehicle having at least two wheels, which comprises: at least two brakes, including wheel cylinders, responsive to a fluid pressure to retard the rotation of the wheels;   a master cylinder actuated by a vehicle operator for generating a controlled fluid pressure;   a source of a predetermined fluid pressure substantially higher than said controlled fluid pressure;   a reservoir for a brake fluid;   at least two pressure modulators in fluid communication with said wheel cylinders, said master cylinder, said source and said reservoir for respectively controlling fluid pressures communicated to respective wheel cylinders;   each of said pressure modulator comprising; valve means responsive to said controlled fluid pressure for controlling fluid communication between said source and associated wheel cylinder and between said reservoir and associated wheel cylinder so that a modulated fluid pressure which is a multiple of said controlled fluid pressure is communicated to associated wheel cylinder during normal braking operation;   electronically controllable first biasing means for biasing said valve means so that said modulated fluid pressure communicated to associated wheel cylinder is reduced upon energization of said first biasing means; and,   electronically controllable second biasing means for controlling fluid communication between said source and associated wheel cylinder and between said reservoir and associated wheel cylinder so that said modulated fluid pressure communicated to associated wheel cylinder is increased upon energization of said second biasing means;     sensing means associated with the wheels for sensing load conditions imposed upon respective wheels and delivering signals in accordance with the load conditions; and,   control means responsive to said signals and cooperating with said first and second biasing means for controlling said first and second biasing means so that fluid pressures communicated from said pressure modulators to respective wheel cylinders are proportioned in accordance with the load conditions at said wheels during braking force proportioning operation of the braking system.   
     
     
       22. A braking system for providing a braking force proportioning function for a vehicle having at least two wheels, which comprises: at least two brakes, including wheel cylinders, responsive to a fluid pressure to retard the rotation of the wheels;   a master cylinder actuated by a vehicle operator for generating a controlled fluid pressure;   a source of a predetermined fluid pressure substantially higher than said controlled fluid pressure;   a reservoir for a brake fluid;   at least two pressure modulators in fluid communication with said wheel cylinders, said master cylinder, said source and said reservoir for respectively controlling fluid pressures communicated to respective wheel cylinders;   each of said pressure modulators comprising; valve means responsive to said controlled fluid pressure for controlling fluid communication between said source and associated wheel cylinder and between said reservoir and associated wheel cylinder so that a modulated fluid pressure which is a multiple of said controlled fluid pressure is communicated to associated wheel cylinder during normal braking operation;   electronically controllable first biasing means for biasing said valve means so that said modulated fluid pressure communicated to associated wheel cylinder is reduced upon energization of said first biasing means; and,   electronically controllable second biasing means for controlling fluid communication between said source and associated wheel cylinder and between said reservoir and associated wheel cylinder so that said modulated fluid pressure communicated to associated wheel cylinder is increased upon energization of said second biasing means;     sensing means associated with the wheels for sensing the rotational speeds of the wheels and delivering signals in accordance with the speeds; and,   control means responsive to said signals and cooperating with said first and second biasing means for sensing a potential wheel skid condition and controlling said first and second biasing means so that fluid pressures communicated from said pressure modulators to respective wheel cylinders are proportioned in accordance with the potential wheel skid conditions at said wheels during braking force proportioning operation of the braking system to avoid the potential wheel skid condition.   
     
     
       23. A braking system for providing a braking force proportioning function for a vehicle having at least two wheels, which comprises: at least two brakes, including wheel cylinders, responsive to a fluid pressure to retard the rotation of the wheels;   a master cylinder actuated by a vehicle operator for generating a controlled fluid pressure;   a source of a predetermined fluid pressure substantially higher than said controlled fluid pressure;   a reservoir for a brake fluid;   at least two pressure modulators in fluid communication with said wheel cylinders, said master cylinder, said source and said reservoir for respectively controlling fluid pressures communicated to respective wheel cylinders;   each of said pressure modulators comprising; valve means responsive to said controlled fluid pressure for controlling fluid communication between said source and associated wheel cylinder and between said reservoir and associated wheel cylinder so that a modulated fluid pressure which is a multiple of said controlled fluid pressure is communicated to associated wheel cylinder during normal braking operation;   electronically controllable first biasing means for biasing said valve means so that said modulated fluid pressure communicated to associated wheel cylinder is reduced upon energization of said first biasing means; and,   electronically controllable second biasing means for controlling fluid communication between said source and associated wheel cylinder and between said reservoir and associated wheel cylinder so that said modulated fluid pressure communicated to associated wheel cylinder is increased upon energization of said second biasing means;     sensing means associated with the wheels for sensing the rotational speeds of the wheels and delivering signals in accordance with the speeds; and,   control means responsive to said signals and cooperating with said first and second biasing means for sensing a potential wheel spin condition and controlling said first and second biasing means so that fluid pressures communicated from said pressure modulators to respective wheel cylinders are proportioned in accordance with the potential wheel skid conditions at said wheels during braking force proportioning operation of the braking system to avoid the potential wheel spin condition.   
     
     
       24. A braking system according to one of claims 1 through 3, wherein said pressure modulator comprises: a housing having therein an inlet chamber, an outlet chamber, a controlled pressure chamber, a low pressure chamber, and an output pressure chamber, said inlet chamber being in fluid communication with said outlet chamber, said low pressure chamber being in fluid communication with said output pressure chamber;   means for communicating said inlet chamber with said source of the predetermined fluid pressure;   means for communicating said outlet chamber with said wheel cylinder;   means for communicating said controlled pressure chamber with said master cylinder;   means for communicating said outlet chamber with said output pressure chamber;   means for communicating said low pressure chamber with said reservoir;   first flow control means for controlling fluid communication between said inlet and outlet chambers;   second flow control means for controlling fluid communication between said output pressure and low pressure chambers;   a valve member responsive to fluid pressure in said outlet and controlled pressure chambers and cooperating with said first and second flow control means for regulating fluid communication between said inlet and outlet chambers and between said output pressure and low pressure chambers, said valve member having for said controlled pressure chamber a pressure receptive area which is a multiple of a pressure receptive area for said outlet chamber member so that fluid pressure communicated to the wheel cylinder is a multiple of the controlled fluid pressure during generation of the controlled fluid pressure; and,   solenoid means, including a solenoid coil mounted to the housing and an armature integral with said valve member, for biasing the valve member against fluid pressure in said controlled pressure chamber.   
     
     
       25. A braking system according to one of claims 4 through 23, wherein said pressure modulator comprises: a housing having therein an inlet chamber, an outlet chamber, a controlled pressure chamber, a low pressure chamber, and an output pressure chamber, said inlet chamber being in fluid communication with said outlet chamber, said low pressure chamber being in fluid communication with said outlet chamber;   means for communicating said inlet chamber with said source of the predetermined fluid pressure;   means for communicating said outlet chamber with said wheel cylinder;   means for communicating said controlled pressure chamber with said master cylinder;   means for communicating said outlet chamber with said output pressure chamber;   means for communicating said low pressure chamber with said reservoir;   a valve member responsive to fluid pressure in said output pressure and controlled pressure chambers for regulating fluid communication between said inlet and outlet chambers and between said outlet and low pressure chambers, said valve member having for said controlled pressure chamber a pressure receptive area which is a multiple of a pressure receptive area for said output pressure chamber member so that fluid pressure communicated to the wheel cylinder is a multiple of the controlled fluid pressure during generation of the controlled fluid pressure;   first solenoid means, including a first solenoid coil mounted to the housing and an armature integral with said valve member, for biasing the valve member against fluid pressure in said controlled pressure chamber to reduce fluid pressure communicated to the wheel cylinder when energized; and,   second solenoid means, including a second solenoid coil mounted to the housing and the armature, for biasing the valve member against fluid pressure in said output pressure chamber to increase fluid pressure communicated to the wheel cylinder when energized.   
     
     
       26. A braking system according to one of claims 4 through 23, wherein said pressure modulator comprises: a housing having therein a stepped bore, an inlet chamber, an outlet chamber, a controlled pressure chamber, and a low pressure chamber, said inlet chamber being in fluid communication with said outlet chamber, said low pressure chamber being in fluid communication with said outlet chamber, said housing also having a valve seat formed between a large diameter section and a small diameter section of said stepped bore;   means for communicating said inlet chamber with said source of the predetermined fluid pressure;   means for communicating said outlet chamber with said wheel cylinder;   means for communicating said controlled pressure chamber with said master cylinder;   means for communicating said low pressure chamber with said reservoir;   a valve member movably received in the large diameter section of the bore and spring-biased into sealing engagement with the valve seat to control fluid communication between the inlet and outlet chambers;   a first sensing piston movably received in the small diameter section of the bore and responsive to fluid pressure in the outlet chamber to cooperate with the valve member to control fluid communication between the outlet and low pressure chambers and to control the position of the valve member;   a second sensing piston responsive to fluid pressure in the controlled pressure chamber to urge the first sensing piston into engagement with the valve member, causing the valve member to move away from the valve seat to permit fluid communication between the inlet and outlet chambers;   said second sensing piston having a pressure receptive area which is a multiple of a pressure receptive area of the first sensing piston so that fluid pressure communicated to the wheel cylinder is a multiple of the controlled fluid pressure during generation of the controlled fluid pressure;   first solenoid means, including a first solenoid coil mounted to the housing and an armature integral with said valve member, for biasing the first sensing piston to move against the force of the second sensing piston to reduce fluid pressure communicated to the wheel cylinder when energized, and,   second solenoid means, including a second solenoid coil mounted to the housing and the armature, for biasing the first sensing piston against fluid pressure in the outlet chamber to increase fluid pressure communicated to the wheel cylinder when energized.   
     
     
       27. A pressure modulator for modulating a fluid pressure communicated to a hydraulic device, which comprises: a housing having therein a controlled pressure chamber communicated with a source of a controlled fluid pressure, an inlet chamber communicated with a source of a predetermined fluid pressure substantially higher than said controlled fluid pressure, an outlet chamber communicated with said hydraulic device, a low pressure chamber communicated with a reservoir for a hydraulic fluid, and an output pressure chamber communicated with said outlet chamber, said low pressure chamber being in fluid communication with said output pressure chamber, said inlet chamber being in fluid communication with said outlet chamber;   first flow control means for controlling fluid communication between said inlet and outlet chambers;   second flow control means for controlling fluid communication between said output pressure and low pressure chambers;   a valve member responsive to fluid pressure in said outlet and controlled pressure chambers and cooperating with said first and second flow control means for regulating fluid communication between said inlet and outlet chambers and between said output pressure and low pressure chambers, said valve member having for said controlled pressure chamber a pressure receptive area which is a multiple of a pressure receptive area for said outlet chamber member so that fluid pressure communicated to the hydraulic device is a multiple of the controlled fluid pressure; and,   solenoid means, including a solenoid coil mounted to the housing and an armature integral with said valve member, for biasing the valve member against fluid pressure in said controlled pressure chamber.   
     
     
       28. A pressure modulator for modulating a fluid pressure communicated to a hydraulic device, which comprises: a housing having therein a controlled pressure chamber communicated with a source of a controlled fluid pressure, an inlet chamber communicated with a source of a predetermined fluid pressure substantially higher than said controlled fluid pressure, an outlet chamber communicated with said hydraulic device, a low pressure chamber communicated with a reservoir for a hydraulic fluid, and an output pressure chamber communicated with said outlet chamber, said inlet chamber being in fluid communication with said outlet chamber, said low pressure chamber being in fluid communication with said outlet chamber;   a valve member responsive to fluid pressure in said output pressure and controlled pressure chambers for regulating fluid communication between said inlet and outlet chambers and between said outlet and low pressure chambers, said valve member having for said controlled pressure chamber a pressure receptive area which is a multiple of a pressure receptive area for said output pressure chamber member so that fluid pressure communicated to the hydraulic device is a multiple of the controlled fluid pressure;   first solenoid means, including a first solenoid coil mounted to the housing and an armature integral with said valve member, for biasing the valve member against fluid pressure in said controlled pressure chamber to reduce fluid pressure communicated to the hydraulic device when energized; and,   second solenoid means, including a second solenoid coil mounted to the housing and the armature, for biasing the valve member against fluid pressure in said output pressure chamber to increase fluid pressure communicated to the hydraulic device when energized.   
     
     
       29. A pressure modulator for modulating a fluid pressure communicated to a hydraulic device, which comprises: a housing having therein a stepped bore, a controlled pressure chamber communicated with a source of a controlled fluid pressure, an inlet chamber communicated with a source of a predetermined fluid pressure substantially higher than said controlled fluid pressure, an outlet chamber communicated with the hydraulic device, and a low pressure chamber communicated with a reservoir for a hydraulic fluid, said inlet chamber being in fluid communication with said outlet chamber, said low pressure chamber being in fluid communication with said outlet chamber, said housing also having a valve seat formed between a large diameter section and a small diameter section of said stepped bore;   a valve member movably received in the large diameter section of the bore and spring-biased into sealing engagement with the valve seat to control fluid communication between the inlet and outlet chambers;   a first sensing piston movably received in the small diameter section of the bore and responsive to fluid pressure in the outlet chamber to cooperate with the valve member to control fluid communication between the outlet and low pressure chambers and to control the position of the valve member;   a second sensing piston responsive to fluid pressure in the controlled pressure chamber to urge the first sensing piston into engagement with the valve member, causing the valve member to move away from the valve seat to permit fluid communication between the inlet and outlet chambers;   said second sensing piston having a pressure receptive area which is a multiple of a pressure receptive area of the first sensing piston so that fluid pressure communicated to the hydraulic device is a multiple of the controlled fluid pressure;   first solenoid means, including a first solenoid coil mounted to the housing and an armature integral with said valve member, for biasing the first sensing piston to move against the force of the second sensing piston to reduce fluid pressure communicated to the hydraulic device when energized; and,   second solenoid means, including a second solenoid coil mounted to the housing and the armature, for biasing the first sensing piston against fluid pressure in the outlet chamber to increase fluid pressure communicated to the hydraulic device when energized.